1. Field of the Invention
The present invention relates to a copying zoom lens system where zooming is performed at a constant conjugate distance, and more particularly, to a three-unit copying zoom lens system of negative, positive, negative configuration.
2. Description of the Prior Art
Conventionally, various types of varifocal lens systems where the conjugate distance is constant, the entire lens system is moved for zooming and the focal length is varied by varying the axial distances have been proposed as copying zoom lens systems.
For example, zoom lens systems comprising from the original side a negative first lens unit, a positive second lens unit and a negative third lens unit and where zooming is performed by varying the distance between the first and second lens units and the distance between the second and third lens units are disclosed in U.S. Pat. Nos. 4,813,773 and 5,270,864, and Japanese Laid-open Patent Application H1-123210.
In the zoom lens systems disclosed by U.S. Pat. No. 4,813,773, the refractive power arrangement of the second lens unit is substantially symmetric, and during zooming, the distance between the first and second lens units and the distance between the second and third lens units are substantially symmetrically varied, so that the refractive power arrangement of the entire lens system is basically symmetric during zooming. In the zoom lens systems disclosed by U.S. Pat. No. 5,270,864 and Japanese Laid-open Patent Application No. H1-123210, the distance between the first and second lens units and the distance between the second and third lens units are asymmetrically varied during zooming so that the refractive power arrangement of the entire lens system is asymmetric during zooming.
However, in the case of the prior art lens systems where the distance between the first and second lens units and the distance between the second and third lens units are substantially symmetrically varied, the refractive power arrangement of the entire lens system is symmetric during zooming, so that the generation amount of lateral chromatic aberration and coma increases although the variation in distortion is small during zooming. The generation of these aberrations are remarkable particularly when the magnification range is increased to 0.5 x to 2.0 x and the F-number is decreased.
In the case of the prior art lens systems where the distance between the first and second lens units and the distance between the second and third lens units are asymmetrically varied, the variation in distortion increases although the generation amount of coma decreases during zooming. The generation of these aberrations are remarkable particularly when the magnification range is increased to 0.5.times. to 2.0.times. and the F-number is decreased.
Thus, conventional three-unit zoom lens systems of negative, positive, negative configuration have both merits and demerits. Particularly, when the magnification range is large, aberrations are not excellently corrected in the entire range.